This application is a 371 application of PCT/EP99/08446 filed Nov. 4, 1999.
The invention relates to a process for the production of optically active xcex1-hydroxyacetals by means of heterogeneous and enantioselective hydrogenation of prochiral xcex1-ketoacetals with platinum as the catalyst and in the presence of a chiral aromatic nitrogen base with at least one basic nitrogen atom adjacent to stereogenic carbon atoms, for example cinchona alkaloids and derivatives thereof.
Optically active xcex1-hydroxyacetals are valuable intermediates in the production of natural compounds [B. T. Cho et al. in Tetrahedron: Asymmetry Vol. 5, No. 7 (1994), pages 1147 to 1150], pharmaceutical active ingredients and pesticides. Cho et al. also describe the asymmetric reduction of xcex1-ketoacetals in homogeneous phase with stoichiometric quantities of a special asymmetric borohydride, namely potassium-9-O-(1,2-isopropylidene-5-deoxy-xcex1-D-xylofuranosyl)-9-boratabicyclo[3.3.1]nonane. H. Takahashi et al. describes in Chemistry Letters (1987), pages 855 to 858 the asymmetric hydrogenation of xcex1-ketopropionic acid methyl ester and 1,1-dimethoxypropan-2-one with chiral rhodium/diphosphine complexes, whereby the optical yields of 1,1dimethoxypropan-2-ol are significantly lower than those of xcex1-hydroxypropionic acid methyl ester. Furthermore, enzymatic reduction processes are known, see for example J. Peters et al. in Tetrahedron: Asymmetry Vol.4, No. 7 (1993), pages 1683 to 1692, and C.-H. Wong et al., J. Am. Chem. Soc. 1985, 107, pages 4028 to 4031. For economic reasons, the above-described processes are not suitable for processes on an industrial scale, primarily because of the high costs in the production of catalysts, which, in addition, can only be separated from the homogeneous reaction mixtures with difficulty and also cannot be reused. In enzymatic or microbial processes, often only low concentrations of substrate may be used, and the necessary reaction control requires complicated reaction equipment.
As long ago as 1979, Orito et al. described that optically active xcex1-hydroxycarboxylates were obtainable in good optical yields by means of hydrogenation of xcex1-ketocarboxylates with platinum metal catalysts in the presence of a cinchona alkaloid. The influence of solvents and other reaction conditions in this hydrogenation Is described by H. U. Blaser et al. in J. of Mol. Cat. 68 (1991), pages 215 to 222. Further studies have shown [see H. U. Blaser et al. in Catalysis Today 37 (1997), pages 441 to 461] that the catalytic hydrogenation system has high substrate specificity. Even the use of xcex1-diketones instead of the xcex1-keto-carboxylates (optical yield, ee up to 95%) leads to considerably lower optical yields (ee only 39 to 50%, see also W. A. H. Vermeer et al. in J. Chem. Soc., Chem. Comm., 1993, pages 1053 to 1054 and M. Studer et al. in J. Chem. Soc., Chem. Comm., 1998, pages 1053). The effect is even more marked when using xcex1-ketomethylethers, and an optical yield of only about 12% ee is obtained (H. U. Blaser et al. in Heterogeneous Catalysis and Fine Chemicals, Elsevier Science Publishers B. V., Amsterdam, 1998, pages 153 to 163).
It has now surprisingly been found that the carbonyl group in xcex1-ketoacetals can be enantio-selectively catalytically hydrogenated with high selectivity and a high yield, with a simultaneously high optical yield (ee to over 95%) even with high concentrations of substrate and even without solvents, if the reaction is carried out in the presence of platinum as the catalyst and in the presence of a soluble or immobilised cinchona alkaloid or derivatives thereof. The catalyst activity is excellent and the catalyst can be simply separated by filtration processes and optionally reused after purification and reactivation. The process is therefore suitable for usage on an industrial scale. This hydrogenation possibility is even more surprising, as xcex1-ketoketals cannot be hydrogenated by this process.
The object of the invention is thus a process for the heterogeneous and enantioselective hydrogenation of prochiral organic xcex1-keto compounds with platinum as the catalyst in the presence of a soluble or immobilised chiral aromatic nitrogen base with at least one basic nitrogen atom adjacent to the stereogenic carbon atoms, which is characterised in that prochiral xcex1-ketoacetals are hydrogenated to optically active xcex1-hydroxyacetals.
Being adjacent to stereogenic carbon atoms may mean, for example, that the basic nitrogen atom is in xcex2- and more preferably in xcex1-position to at least one stereogenic carbon atom.
The prochiral xcex1-ketoacetals in question may be saturated or unsaturated, open-chained or cyclic compounds, which contain preferably 5 to 30, most preferably 5 to 20 carbon atoms, which are unsubstituted or substituted by radicals that are stable under the hydrogenation conditions. The carbon chain may be interrupted by hetero atoms preferably from the group xe2x80x94Oxe2x80x94, xe2x95x90Nxe2x80x94 and xe2x80x94NRxe2x80x2xe2x80x94, wherein Rxe2x80x2 is H, C1-C8-alkyl, preferably C1-C4-alkyl, C5- or C6-cycloalkyl, C6-C10-aryl, for example phenyl or naphthyl, phenyl or phenylethyl.
The xcex1-ketoacetals preferably correspond to formula I 
wherein R1, R2 and R3, independently of one another, signify a monovalent, saturated or unsaturated, aliphatic radical with 1 to 12 carbon atoms, a saturated or unsaturated cyclo-aliphatic radical with 3 to 8 carbon atoms, a saturated or unsaturated heterocycloaliphatic radical with 3 to 8 ring members and one or two hetero atoms from the group O, N and NRxe2x80x2, a saturated or unsaturated cycloaliphatic-aliphatic radical with 4 to 12 carbon atoms, a saturated or unsaturated heterocycloaliphatic-aliphatic radical with 3 to 12 carbon atoms and one or two hetero atoms from the group O, N and NRxe2x80x2, an aromatic radical with 6 to 10 carbon atoms, a heteroaromatic radical with 4 to 9 carbon atoms and one or two hetero atoms from the group O and N, an aromatic-aliphatic radical with 7 to 12 carbon atoms or a heteroaromatic-aliphatic radical with 5 to 11 carbon atoms and one or two hetero atoms from the group O and N, whereby Rxe2x80x2 is H, C1-C8-alkyl, preferably C1-C4-alkyl, C5- or C6-cyclo-alkyl, C6-C10-aryl, for example phenyl or naphthyl, phenyl or phenylethyl, R1 and R2 together are C1-C6-alkylene or C3-C8-1,2-cycloalkylene; or C2-C4-alkylene or C3-C8-cycloalkylene which are condensed with 1,2-phenylene, and R3 has the above-mentioned significances,
R2 and R3 together signify C1-C6-alkylene, C1-C8-alkylidene, C3-C8-1,2-cycloalkylene, C3-C8-cycloalkylidene, benzylidene, 1,2-phenylene, 1,2-pyridynylene, 1,2-naphthylene; or C3-C4-alkylene or C3-C8-1,2-cycloalkylene which are condensed with 1,2-cycloalkylene or with 1,2-phenylene, and R1 has the above-mentioned significances,
and R1, R2 and R3 are unsubstituted or substituted by one or more identical or different radicals selected from the group C1-C4-alkyl, C2-C4-alkenyl, C1-C4-alkoxy, C1-C4-halogen-alkyl, C1-C4-hydroxyalkyl, C1-C4-alkoxymethyl or -ethyl, C1-C4-halogenalkoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyl, phenylethyloxy, halogen, xe2x80x94OH, xe2x80x94OR4, xe2x80x94OC(O)xe2x80x94R4, xe2x80x94NH2, xe2x80x94NHR4, xe2x80x94NR4R5, xe2x80x94NHxe2x80x94C(O)xe2x80x94R4, xe2x80x94NR4xe2x80x94C(O)xe2x80x94R4, xe2x80x94CO2R4, xe2x80x94CO2xe2x80x94NH2, xe2x80x94CO2xe2x80x94NHR4, xe2x80x94CO2xe2x80x94NR4R5, wherein
R4 and R5, independently of one another, signify C1-C4-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.
The heterocyclic radicals are bonded by a ring carbon atom to the oxygen atoms or the carbon atom of the carbonyl group in formula I.
Preferred substituents are methyl, ethyl, n- and i-propyl, n- and t-butyl, vinyl, allyl, methyloxy, ethyloxy, n- and i-propyloxy, n- and t-butyloxy, trifluoromethyl, trichloromethyl, xcex2-hydroxyethyl, methoxy- or ethoxymethyl or -ethyl, trifluoromethoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyloxy, phenylethyl, halogen, xe2x80x94OH, xe2x80x94OR4, xe2x80x94OC(O)R4, xe2x80x94NH2, xe2x80x94NHR4, xe2x80x94NR4R5, xe2x80x94NHxe2x80x94C(O)xe2x80x94R4, xe2x80x94NR4xe2x80x94C(O)xe2x80x94R4, xe2x80x94CO2R4, xe2x80x94CO2xe2x80x94NH2, xe2x80x94CO2xe2x80x94NHR4, xe2x80x94CO2xe2x80x94NR4R5, wherein R4 and R5, independently of one another, signify C1-C4-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.
The aliphatic radical in question is preferably alkyl, which may be linear or branched, and preferably contains 1 to 8, most preferably 1 to 4 carbon atoms, or preferably alkenyl or alkynyl, which may be linear or branched and preferably contain 2 to 8, most preferably 2 to 4 carbon atoms. If R2 and R3 are alkenyl or alkynyl, the unsaturated bond is preferably in xcex2-position to the oxygen atom. Examples are methyl, ethyl, n- and i-propyl, n-, i- and t-butyl, pentyl, i-pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl, vinyl, allyl, ethynyl and propargyl. One preferred group of aliphatic radicals is methyl, ethyl, n- and i-propyl, n-, i- and t-butyl.
The cycloaliphatic radical in question is preferably cycloalkyl or cycloalkenyl with preferably 3 to 8, most preferably 5 or 6 ring carbon atoms. Some examples are cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl, as well as cyclopentenyl, cyclohexenyl and cyclohexadienyl. Cyclopentyl and cyclohexyl are preferred in particular.
The heterocycloaliphatic radical in question is preferably heterocycloalkyl or heterocyclo-alkenyl with preferably 3 to 6 carbon atoms, 4 to 7 ring members, and hetero atoms selected from the group xe2x80x94Oxe2x80x94 and xe2x80x94NRxe2x80x2xe2x80x94, wherein Rxe2x80x2 is H, C1-C8-alkyl, preferably C1-C4-alkyl, C5- or C6-cycloalkyl, C6-C10-aryl, for example phenyl or naphthyl, phenyl or phenylethyl. Some examples are pyrrolidinyl, pyrrolinyl, tetrahydrofuranyl, dihydrofuranyl and piperazinyl.
The cycloaliphatic-aliphatic radical in question is preferably cycloalkyt-alkyl or -alkenyl with preferably 3 to 8, most preferably 5 or 6 ring carbon atoms, and preferably 1 to 4, or 2-4, most preferably 1 or 2, or 2 or 3, carbon atoms in the alkyl group or alkenyl group. Some examples are cyclopentyl- or cyclohexylmethyl or -ethyl and cyclopentyl or cyclohexyl-ethenyl.
The heterocycloaliphatic-aliphatic radical in question is preferably heterocycloalkyl-alkyl or -alkenyl with preferably 3 to 6 carbon atoms, 4 to 7 ring members, and hetero atoms selected from the group xe2x80x94Oxe2x80x94 and xe2x80x94NRxe2x80x2xe2x80x94, wherein Rxe2x80x2 is H, C1-C8-alkyl, preferably C1-C4-alkyl, C5- or C6-cycloalkyl, C6-C10-aryl, for example phenyl or naphthyl, phenyl or phenylethyl, and preferably 1 to 4, most preferably 1 or 2, carbon atoms in the alkyl group, or 2 to 4 and most preferably 2 or 3 carbon atoms in the alkenyl group. Examples are pyrrolidinylmethyl or -ethyl or -ethenyl, pyrrolinylmethyl or -ethyl or -ethenyl, tetrahydrofuranylmethyl or -ethyl or -ethenyl, dihydrofuranylmethyl or -ethyl or -ethenyl, and piperazinylmethyl or -ethyl or -ethenyl.
The aromatic radicals in question are especially naphthyl and in particular phenyl.
The aromatic-aliphatic radicals in question are preferably phenyl- or naphthyl-C1-C4-alkyl or-C2-C4-alkenyl. Some examples are benzyl, naphthylmethyl, xcex2-phenylethyl and xcex2-phenyl-ethenyl.
The heteroaromatic radicals in question are preferably 5- or 6-membered, optionally condensed ring systems. Some examples are pyridinyl, pyrimidinyl, pyrazinyl, pyrrolyl, furanyl, oxazolyl, imidazolyl, benzofuranyl, indolyl, benzimidazolyl, quinolinyl, isoquinolinyl, quinazolinyl, quinoxalinyl.
The heteroaromatic-aliphatic radicals in question are preferably 5- or 6-membered, optionally condensed ring systems, which are bonded by one of their carbon atoms to the free bond of an alkyl group or alkenyl group, whereby the alkyl group contains preferably 1 to 4, most preferably 1 or 2 carbon atoms, and the alkenyl group contains preferably 2 to 4, most preferably 2 or 3 carbon atoms. Some examples are pyridinylmethyl or -ethyl or -ethenyl, pyrimidinylmethyl or -ethyl or -ethenyl, pyrrolylmethyl or -ethyl or -ethenyl, furanyl-methyl or -ethyl or -ethenyl, imidazolylmethyl or -ethyl or -ethenyl, indolylmethyl or -ethyl or -ethenyl.
More preferred compounds of formula I include those wherein
R1, R2 and R3, independently of one another, signify linear or branched C1-C8-alkyl, C4-C7-cycloalkyl or C4-C6-hetreocycloalkyl with hetero atoms from the group O and N, C6-C10-aryl or C4-C9-heteroaryl with hetero atoms from the group O and N, C4-C7-cycloalkyl-C1-C4-alkyl or C3-C6-heterocycloalkyl-C1-C4-alkyl with hetero atoms from the group O and N, C6-C10-aryl-C1-C4-alkyl or C4-C9-heteroaryl-C1-C4-alkyl with hetero atoms from the group O and N, R1 and R2 together signify C1-C4-alkylene or C4-C7-1,2-cycloalkylene; or C2-C4-alkylene or C4-C7-cycloalkylene condensed with 1,2-phenylene, and R3 has the above-mentioned significances, R2 and R3 together signify C1-C4-alkylene, C1-C4-alkylidene, C4-C7-1,2-cycloalkylene, C4-C7-cycloalkylidene, benzylidene, 1,2-phenylene, 1,2-pyridinylene, 1,2-naphthylene; or C3-C4-alkylene or C4-C7-cycloalkylene condensed with 1,2-cycloalkylene or with 1,2-phenylene, and R1 has the above-mentioned significances, whereby R1, R2 and R3 are unsubstituted or substituted by one or more, identical or different radicals selected from the group C1-C4-alkyl, C1-C4-alkoxy, C1-C4-halogenalkyl, C1-C4-hydroxyalkyl, C1-C4-alkoxymethyl or -ethyl, C1-C4-halogenalkoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyl, phenylethyloxy, halogen, xe2x80x94OH, xe2x80x94OR4, xe2x80x94OC(O)R4, xe2x80x94NH2, xe2x80x94NHR4, xe2x80x94NR4R5, xe2x80x94NHxe2x80x94C(O)xe2x80x94R4, xe2x80x94NR4xe2x80x94C(O)xe2x80x94R4, xe2x80x94CO2R4, xe2x80x94CO2xe2x80x94NH2, xe2x80x94CO2xe2x80x94NHR4, xe2x80x94CO2xe2x80x94NR4R5, wherein R4 and R5, independently of one another, signify C1-C4-alkyl, cyclohexyl, phenyl or benzyl.
One preferred sub group of the compounds of formula I is those wherein R1, R2 and R3, independently of one another, signify linear or branched C1-C4-alkyl, C2-C4-alkenyl, C5-C6-cycloalkyl, phenyl, phenylethenyl, C5-C6-cycloalkyl-C1-C2-alkyl or C6-C10-aryl-C1-C2-alkyl, R1 and R2 together signify C1-C3-alkylene or C5-C6-1,2-cycloalkylene, R2 and R3 together signify C2-C4-alkylene, C1-C4-alkylidene, C5-C6-1,2-cycloalkylene, C5-C6-cycloalkylidene, benzylidene, 1,2-phenylene, whereby R1, R2 and R3 are unsubstituted or substituted as above.
One especially preferred sub group of the compounds of formula I is those wherein R1 signifies C1-C4-alkyl, C2-C4-alkenyl, cyclohexyl, phenyl, benzyl, phenylethyl or phenyl-ethenyl, R2 and R3, independently of one another, signify linear or branched C1-C4-alkyl, cyclohexyl, phenyl, benzyl or phenylethyl, R1 and R2 together signify C2-C3alkylene or 1,2-cyclohexylene, R2 and R3 together signify C2-C3-alkylene, C1-C4-alkylidene, 1,2-cycohexylene, cyclohexylidene, benzylidene or 1,2-phenylene, whereby R1, R2 and R3 are unsubstituted or substituted by methyl, ethyl, n- and i-propyl, n- and t-butyl, vinyl, allyl, methyloxy, ethyloxy, n- and i-propyloxy, n- and t-butyloxy, trifluoro-methyl, trichloromethyl, xcex2-hydroxyethyl, methoxy- or ethoxymethyl or -ethyl, trifluoro-methoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyloxy, phenylethyl, halogen, xe2x80x94OH, xe2x80x94OR4, xe2x80x94OC(O)R4, xe2x80x94NH2, xe2x80x94NHR4, xe2x80x94NR4R5, xe2x80x94NHxe2x80x94C(O)xe2x80x94R4, xe2x80x94NR4xe2x80x94C(O)xe2x80x94R4, xe2x80x94CO2R4, xe2x80x94CO2xe2x80x94NH2, xe2x80x94CO2xe2x80x94NHR4, xe2x80x94CO2xe2x80x94NR4R5, wherein R4 and R5, independently of one another, signify C1-C4-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.
xcex1-ketoacetals are known or may be produced in a manner known per se by reacting alcohols with xcex1-ketoaldehydes whilst removing the reaction water.
The ketoacetals, especially those of formula I, are hydrogenated to chiral secondary alcohols of formula II, 
wherein R1, R2 and R3 have the significances given above and the symbol * primarily denotes the R- or S-form of one of the stereoisomers.
Platinum catalysts are known, have been described many times and are commercial. Platinum may be used both in metal form, for example as a powder, and, preferably, as platinum metal applied to finely-dispersed carriers. Suitable carriers are for example carbon, metal oxides, for example SiO2, TiO2, Al2O3, metal salts, and natural or synthetic silicates. The catalyst in question may also be a platinum colloid. The amount of platinum metal on the carrier may be for example 1 to 10, preferably 3 to 8% by weight, based on the carrier. Prior to usage, the catalysts may be activated by means of treatment with hydrogen at an elevated temperature or by ultrasound.
Chiral and aromatic nitrogen bases as modifiers for the platinum-catalysed enantoselective hydrogenation are similarly known, and are described for example by H.-U. Blaser et al. in Catalysis Today 37 (1997), pages 441 to 463. The nitrogen bases that are suitable are, in particular, those containing an aromatic or heteroaromatic, mononuclear or multinuclear ring, preferably mono- to trinuclear ring, optionally in combination with condensed cycloaliphatic or heterocycloaliphatic rings, whereby the basic N-atom(s) is or are bound in xcex2- and preferably in xcex1-position to a chiral carbon atom, and are ring members of a chiral N-cycloheteroaliphatic ring, or are bound to a ring by a chiral C1- or C2-group.
Preference is given to cinchona alkaloids and derivatives thereof. They may correspond, for example, to formula III 
wherein R signifies H, C1-C4-alkyl, C1-C4-alkyl-C(O)C, C1-C4-hydroxyalkyl-C(O)Oxe2x80x94, phenyl-C(O)Oxe2x80x94 or benzyl-C(O)Oxe2x80x94, R6 is H, C1-C4-alkyl, C1-C4-hydroxyalkyl, or C2-C4-alkenyl, and the symbol * denotes the R- or S-form of the stereo centres. Preferred cinchona alkaloids are those in which, in formula III, R6 signifies H, methyl, ethyl or vinyl, and R is H, methyl, ethyl and acetyl.
The choice of nitrogen base determines which of the enantiomeric xcex1-hydroxyacetals is primarily formed.
The catalyst metal may be used for example in an amount of 0.01 to 10, preferably 0.05 to 50, most preferably 0.1 to 10% by weight, based on the xcex1-ketoacetal employed, whereby amounts of 0.1 to 5% by weight, or 0.1 to 1% by weight, are generally sufficient.
The nitrogen base is used for example in an amount of 0.1 to 1000, preferably 1 to 500, most preferably 10 to 200% by weight, based on the platinum metal employed. The nitrogen base may be added to the reaction vessel together with the platinum metal catalyst, or the platinum metal catalyst may already be impregnated with the nitrogen base, for example a cinchona alkaloid.
Hydrogenation Is preferably carried out at a hydrogen pressure of up to 200 bar, more preferably up to 150 bar, most preferably 10 to 100 bar.
The reaction temperature may be for example xe2x88x9250 to 100xc2x0 C., more preferably 0 to 50xc2x0 C., most preferably 0 to 35xc2x0 C.
The reaction may be carried out without or in an inert solvent. Suitable solvents are for example aliphatic, cycloaliphatic and aromatic hydrocarbons (pentane, hexane, petroleum ether, cyclohexane, methylcyclohexane, benzene, toluene, xylene), ethers (diethyl ether, dibutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, tetrahydrofuran, dioxane), alcohols (methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl or monoethyl ether, diethylene glycol monomethyl or monoethyl ether), ketones (acetone, methyl isobutyl ketone), carboxylates and lactones (ethyl or methyl acetate, valerolactone), N-substituted carboxylamides and lactams (dimethylformamide, N-methylpyrrolidone) and carboxylic acids (acetic acid, propionic acid, butyric acid). The optimum yield may be influenced by the choice of solvent. Carboxylic acids such as acetic acid have proved to be particularly suitable for this purpose.
The process according to the invention may be carried out, for example, whereby the catalyst with the nitrogen base is placed in an autoclave, optionally with a solvent, then the xcex1-ketoacetal is added, afterwards the air is displaced with an inert gas, for example noble gases, hydrogen pressure is applied, and then the reaction is started, if required whilst stirring or agitating, and hydrogenation takes place until the up-take of hydrogen is no longer observed. The xcex1-hydroxyacetal formed may be isolated and purified by conventional methods, for example distillation, crystallisation and chromatography.
The xcex1-hydroxyacetals that can be produced according to the invention are valuable intermediates in the production of natural active ingredients [B. T. Cho et al. in Tetrahedron: Asymmetry Vol. 5, No. 7 (1994), pages 1147 to 1150], and synthetic pharmaceutical active ingredients and pesticides. The obtainable xcex1-hydroxyacetals may be first converted into derivatives by known methods, and these may then be used as intermediates in the production of active ingredients. Acidic hydrolysis leads to 1,4-dioxanes or the corresponding aldehydes, which are either hydrogenated to 1,2-diols with a secondary optically active hydroxyl group, or reacted with amines in the presence of phenylboric acids to form optionally substituted optically active 1-phenyl-1-amino-2-hydroxyalkanes. After protection of the OH group, e.g. by means of a reaction with benzyl bromide, a reaction with strong acids will produce the hydroxyl-protected aldehydes, which may be hydrogenated to 1,2-diols, or may be converted by oxidation (for example with chromium trioxide) and removal of the protecting group to R- or S-xcex1-hydroxycarboxylic acids.